Due to structural similarities to graphene, hexagonal boron nitride (h-BN) nanosheets have become an extremely desirable material over the last few years. While many synthetic efforts have been made to combine these two materials for several electronic applications, recently few-layer h-BN nanosheets have been successfully developed as substrates for graphene. Furthermore, h-BN nanosheets are useful in a number of versatile applications due to unique inherent physical properties. h-BN exhibits excellent chemical and mechanical stability and it is thermally conductive and electrically insulating with a wide band gap (5-6 eV) as well. While h-BN is being used as an electrical insulator in thermally conductive materials, composites of h-BN have been the preferred species in aircrafts for their radiation shielding properties.
Although the development of graphene and graphene composites are continuously advancing, few-layer h-BN nanostructures are comparatively less investigated due to synthetic difficulties. Preparation of h-BN nanostructures involves one of the two common approaches, top-down or bottom-up. The top-down process mainly includes mechanical or chemical exfoliation of h-BN nanosheets from bulk h-BN. Although it is one of the most common techniques currently used to produce nanomaterials on a large scale, the major disadvantage of this method is the imperfection of surface structure imparted during the process. On the contrary, the bottom-up approach yields nanostructures with minimal defects and superior chemical homogeneity. Typically, chemical vapor deposition (CVD) is used as a bottom-up approach. However, the requirement of substrate and extreme reaction conditions makes this synthetic route less desirable. Involvement of catalysts in a CVD process not only restricts industrial scaling up of the method, but also yields products with metal impurities.
Recent reports describes the production of defect-free h-BN nanofibers and nanoparticles at 1000 to 1250° C. in a tube furnace (Lin et al., Solid State Sciences, 2007, 9, 1099; and Lin et al., Nanotechnology, 2011, 22, 215603). These reports describe the preparation of an “intermediate material” containing a “precursor”, ammonium chloride (NH4Cl) and potassium chloride (KCl); the intermediate material was prepared by mixing and heating solutions of potassium borohydride (KBH4) and ammonium chloride, followed by distilling off the water. (Although the precursor was isolated by washing the intermediate material with ice water to remove ammonium chloride and potassium chloride, it was not characterized.) The intermediate material was heated under flowing nitrogen at atmospheric pressure to 1000° C. or 1250° C. to form h-BN nanoparticles or nanofibers, respectively. The size of nanoparticles ranged from 30 to 90 nm, while the nanofibers had a diameter of 100 to 500 nm and typically a length greater than 5 μm (5000 nm). No h-BN nanosheets were described.
Consequently, a controlled cost-effective synthetic methodology to prepare boron nitride (BN) nanosheets on a large scale is warranted. There have been no reports to date on a bottom-up methodology that avoids catalysts altogether in producing pristine few-layer BN nanosheets.